Referring to FIG. 1, a pilot fuel can be employed in Diesel-cycle internal combustion engine 100 to ignite a main fuel when the main fuel has a high octane number making it difficult to auto-ignite. Pilot fuel is introduced into combustion chambers (not shown) where it auto-ignites because of the pressure and temperature environment, and the subsequent combustion of pilot fuel creates an environment that is suitable to ignite the main fuel. Fuel injectors that introduce both the pilot and main fuels into the combustion chambers, such as fuel injector 110, can employ the pilot fuel in fluid seals (not shown) for sealing the main fuel inside the injector body, for example when the pilot fuel is a liquid fuel like diesel and the main fuel is a gaseous fuel like natural gas. A gaseous is defined as a fuel in a gas state at a pressure of 1 atmosphere and a temperature of 25 degrees Celsius within this specification. Only one such fuel injector is shown in engine 100, but as would be understood by those familiar with the technology there can be one or more fuel injectors. The Applicant's own U.S. Pat. No. 7,124,959, issued Oct. 24, 2006 to Baker et al., hereinafter the '959 reference, discloses a dual fuel injection valve that injects separately and independently two different fuels, which can be employed to introduce the pilot and main fuels, and employs fluid seals to seal main fuel cavities within the injector from other cavities. The differential pressure between the pilot fuel and the main fuel (referred to herein as the bias) is maintained within a range of tolerance for the fluid seals and injector valve 110 to function correctly. One technique for regulating the bias is to employ dome loaded regulator 120 that uses pilot fuel pressure in piping 130 as a loading mechanism for regulating main fuel pressure in piping 140, which is equal to pilot fuel pressure minus the bias. The pilot fuel is pressurized by pilot pumping apparatus 150 and delivered to fuel injector 110 and dome loaded regulator 120 at pilot fuel injection pressure through a pilot fuel circuit comprising common rail 160 and piping 130. Pressure sensor 165 sends signals representative of pilot fuel injection pressure in common rail 160 to control 250. Pilot fuel pumping apparatus 150 can comprise an inlet metering valve (not shown) and a common rail pump (not shown), as is known by those familiar with the technology. The main fuel is pressurized by main pumping apparatus 170 in main fuel supply system 180 and delivered to dome loaded regulator 120 where its pressure is reduced and then delivered to fuel injector 110 through a main fuel circuit comprising piping 140 and main fuel rail 190. Pressure sensor 195 sends signals representative of main fuel injection pressure in main fuel rail 190 to control 250.
In certain operating modes of the internal combustion engine the fuelling commands (injection amount per stroke) for the pilot and main fuels are reduced to zero, as illustrated in FIG. 2 where main fuelling command 300 and pilot fuel command 310 are reduced to zero at time T1. Before time T1 is a motoring operating mode when fuel is injected and ignited in the combustion chamber engine 100. After time T1, is a non-motoring operating mode when no fuel is introduced into the combustion chamber. The fuelling command can reduce to zero when, for example, a vehicle driven by engine 100 decelerates. During zero fuelling command, main injection command signals sent through wire 230 and pilot injection command signals sent through wire 240 from electrical controller 250 (seen in FIG. 1) to actuate fuel injector 110 are stopped such that no fuel is introduced to the combustion chambers. Additionally, pilot pumping apparatus 150 that pressurizes the pilot fuel is commanded by controller 250 to stop, suspend or shut-off pilot flow to the pilot fuel circuit. The pilot fuel circuit becomes a closed volume of pressurized fluid that goes into a state of hydraulic lock, since no pilot fuel can enter or leave this circuit. Pilot pumping apparatus 150 can only control how much fluid is added to the pilot fuel circuit and does not allow any back flow. Piston 205 inside dome loaded regulator 120 is prevented from decreasing the volume of the pilot fuel circuit when the pilot fuel is an incompressible liquid, such as diesel. Valve 200 inside dome loaded regulator 120, which is connected with piston 205 and regulates the flow of main fuel between pumping apparatus 170 and piping 140, is prevented from closing when the pilot fuel circuit is hydraulically locked causing loss of main fuel pressure regulation. Referring to FIG. 3, loss of main fuel pressure regulation results in main fuel injection pressure 320 (in rail 190 as seen in FIG. 2) increasing towards pilot fuel injection pressure 330 (in rail 160), reducing the bias between these two fuels. The fluid seals within fuel injector 110 begin to leak main fuel from the main fuel cavities into cavities filled with pilot fuel when the main fuel pressure rises above the pilot fuel pressure (negative bias) caused by the malfunctioning dome loaded regulator. Negative bias is illustrated in FIG. 3 between the pilot and main fuels after time T2.
Fuel injection and/or combustion does not occur as expected when the fuelling command is increased from zero under conditions of negative bias. Fuel injector 110 may fail to inject the pilot fuel or the main fuel, or both fuels, and if fuel is injected reduced ignition performance can occur. Injection and ignition problems are caused by the displacement of pilot fuel by main fuel inside fuel injector 110, which can prevent pilot and main injection valves from opening and/or incorrect injections of both pilot and main fuel. Only after several injection events does the bias return to within the predetermined range of tolerance and main fuel is cleared from pilot fuel cavities within fuel injector 110, after which injection and combustion successfully occurs. Another consequence of negative bias is contamination of pilot fuel drain circuit 210 with main fuel. This is caused by main fuel draining from pilot fuel cavities within fuel injector 110 into pilot fuel drain circuit 210 during injection events. Pilot fuel drain circuit 210 returns pilot fuel to supply tank 220 which for some known pilot fuels, such as diesel, is not designed to be sealed under all conditions (such as when being refilled). The contamination of drain circuit 210 with main fuel results in an increase in unburned hydrocarbon emissions when the main fuel is a gaseous fuel.
U.S. Pat. No. 5,711,274, issued Jan. 27, 1998 to Eugen Drummer (the Drummer reference), discloses a technique of reducing a high pressure in a common rail after an engine associate with the common rail is shut down. Previous common rail injection systems had the disadvantage of the high fuel pressure remaining in the system for a long time after the engine was shut down, which made maintenance and repair work on the fuel injection system quite dangerous. Drummer teaches to activate a magnetic valve that actuates a control valve inside a fuel injector that, depending on the structural design of the fuel injector, briefly relieves pressure in a control chamber at the valve member or briefly increases the pressure on a pressure chamber acting upon the valve member in the opening direction, followed by refilling of the control chamber or pressure relief of the pressure chamber, such that the high pressure can be reduced continuously via a relief line into a supply tank.
The present method and apparatus provide a technique for operation of a regulator that regulates the pressure of one fuel based on the pressure of another fuel.